Edwin,
This is awesome! I've done something sort of similar a while ago: New project: Arduinoponics
using OpenWRT on a router and communicating to an Arduino over serial. It looks from your wiki that SPI is currently used? Interesting that the board can be programmed as well as just interacted with.
That's an awesome thing for enabling remote firmware upgrades to the MCU if this setup is tucked away somewhere.
But perhaps rather an doughterboard, since this could flash anyway, perhaps an Arduino (well ATMega328 really) could simply be added to the PCB itself, SPI vias etched out on the PCB, and its GPIO exposed externally somehow? I don't know how often you'd really need to replace a daughterboard if this setup could allow you to upgrade firmware via the router itself.
BTW, can your wireless SoC allow for the router to act as a Wifi client and expose a bridged AP at the same time? This is a really great project!